Technical Field
The present invention relates to a contactless distance sensor having improved accuracy and precision, in particular in measuring the distance of moving target objects. Furthermore, the invention relates to an operating method of such a sensor.
Description of the Related Art
Devices or sensors for measuring the distance of objects are known as “contactless” when, in order to carry out the aforementioned measurement, there is no physical contact between the object for which it is wished to measure the distance and the sensor. This type of sensor is generally used in various applications from process automation to quality control in test beds, pneumatic cylinders, in engineering, etc. A sub-class of these sensors is called proximity sensors, usually being able to detect the presence of objects in the immediate vicinity of the “sensitive side” of the sensor itself, of course also in this case without there being any actual physical contact.
Distance or proximity sensors can use different measurement principles. In any case, generally, the sensor emits electromagnetic radiation or sound waves that hit the object for which it is wished to measure the distance or presence, and the sensor measure the differences in the return radiation with respect to that emitted. Typical examples of such sensors are sensors based on triangulation, sensors which are based on the Doppler effect, laser rangefinders, capacitive sensors, etc.
In a further type of distance sensor, the measurement of the distance of an object is obtained from the measurement of the time that a given signal takes to reach the object and come back. Such sensors are called Time Of Flight (TOF) sensors. However, the time taken by light to “come back” is not generally measured directly. In general, modulated pulses of light are sent, for example as a sinusoid, and the phase of the light signal sent and the phase of the light signal reflected and detected are both measured. These sensors can be very precise and accurate.
The distance within which distance or proximity sensors are able to detect objects is defined as nominal range (or sensitive field). Some sensors have an adjustment system so as to be able to calibrate the detection distance.
The field of application of distance or proximity sensors can be divided into static measurements, in which the target object of measurement stays still during the entire period of observation, and dynamic measurements, in which the target object can move during the measurement. In both cases, the sensor must provide a response within predetermined time periods. Generically, static measurements can be defined as those measurements the target object of which stays still for times of more than a second, whereas dynamic measurements can be defined as those measurements where the target object stays immobile for much less than one second.
The measurement of moving objects is particularly important in industry. Contactless measuring sensors are frequently used for measurements of target objects moving along the detection trajectory of the measurement or with target objects that are inserted from one side on a plane perpendicular to the measurement trajectory. These application conditions require a sensor that has a fast response time, suitable for quickly detecting the new position and informing the control system to which it is connected.
In fact, by its very nature, the position information of a moving target object is per sé indeterminate, or rather, it is of little use to know precisely the position (and thus the distance) of the target object at a certain instant if it is no longer in the same position at the next instant. Vice-versa it is very useful and desirable to have a highly precise measurement for stationary or slow-moving target objects.
The measurement of moving target objects requires a quick response so that the associated control system can react as quickly as possible. A typical example in the field of logistics are traveling lifts or rack feeders, mobile goods storage systems that, moving along rigid tracks, make it possible to deposit pallets of goods in suitable housings spread along the tracks that make up the automatic store. The measurement sensors are positioned along the trajectory of the rack feeder in order to detect their position along the track and lock them at the housing where the goods are to be deposited. The need to optimize the loading and unloading operations is such that the response speed of the sensor is one of the most important characteristics, but it is equally important to ensure precise positioning in front of the loading bay when it is identified.
In some of the sensors currently on the market, the user—that is whoever uses the sensor—is given the possibility to choose which of the two methods to use, i.e., the user can typically choose between a “precise” mode suitable for stationary or almost-stationary objects, a “medium precision mode”, suitable for objects that move slowly or in a limited ranged of distances, and a “fast” mode, with immediate response time, but having little precision, suitable for objects moving about a lot, at the limit that suddenly enter or leave the measurement field.
The user usually is not able to modify the behavior of the sensor, unless by interrupting the measurement and resetting the operating mode.
In order to increase the precision of the measurement in a distance or proximity sensor, it is known in the field of reference to carry out the acquisition of an ever greater number of samples. In other words, it is not just a single measurement of the distance or presence of an object in the sensitive field of the sensor that is carried out, but N of them that are memorized and an average of which will then be taken.
It is known from statistics that the center of the statistical distribution of the measured values represents the mean value, or most probable value of the measurement.
However, the Applicant has noted how this solution is not optimal, in particular for the measurement of moving target objects. For static measurements, for the same processing times of the measurement device, the acquisition of a large number of measurements, as described above, can be sufficient to obtain the required precision, since the precision of the required measurement can be obtained without particular time constraints, simply obtaining a number N of measurements with N sufficiently large. Such a technique is also slow and imprecise for a moving object: for dynamic measurements giving priority to one characteristic could automatically worsen the other and this is often not tolerated; in other words, giving priority to precision makes the device slow, giving priority to response speed, for example emitting the last measurement value acquired in the case of moving objects as an estimation of the distance, makes the device imprecise.
Moreover, the fact that the operating mode of the sensor used to carry out the measurement, for which the output value of the sensor is for example either the average of N measurements, or the last measurement obtained, depends on the choice of the user, can make the measurement inaccurate, in the case of an incorrect choice among the operating modes available or a change in the measurement situation from a stationary object to a moving object, or vice-versa, without the user realizing. Moreover, the fact that the user must make the choice manually requires continuous supervision of the sensor by the user, wasting time and money.